The present invention concerns a muffler for attenuating noise produced by a pneumatic device. More particularly, it relates to a reduced-sized muffler incorporating an acoustic absorption insert for use with a pneumatic device having limited available area for muffler placement.
A wide variety of different devices are pneumatically controlled and/or actuated. Such devices include processing equipment incorporating one or more pneumatic valve banks, pneumatic robotic applications, pneumatic testing equipment, hand-held pneumatic tools, pumps, etc. Basically, flow of a pressurized fluid, normally air, is used to actuate or maneuver a mechanism, such as a linkage arm, resulting in a desired output. Depending upon the particular application, one or more pneumatic valves are typically utilized to direct the forced air to a desired location within the device, as well as to release the air through an exhaust port. Because the air is pressurized and the exhaust port relatively small, the exhausted air is normally traveling at a high velocity. As the high velocity air flows into relatively still air, the airflow becomes turbulent. Eddies associated with the now turbulent airflow generate pressure fluctuations, resulting in exhaust noise.
Depending upon the particular application, the exhaust noise may rise to an unacceptable level, potentially leading to noise-induced hearing loss. As a point of reference, United States standards require hearing protection for individuals exposed to continuous noise levels in excess of 85 decibels (dB) over an 8-hour period. International standards require hearing protection for noise levels in excess of 80 dB over an 8-hour period. Notably, exhaust noise at less than 80 dB, or intermittent noise at levels greater than 80 dB, can be equally irritating and harmful.
Various techniques can be employed to minimize the effect of exhaust noise produced by a pneumatic device. For example, an individual working in close proximity to the device may be provided with hearing protection. Unfortunately, the operator may forget to wear the hearing protection, or may simply choose not to use it due to perceived inconveniences. Additionally, other nearby workers or visitors who do not wear hearing protection will be subjected to the same noise-related concerns. Alternatively, a sound barrier or enclosure may be placed about the device. In many instances, however, this approach is not viable from both a cost standpoint and because an external barrier may unduly impede proper device operation. A third, more practical approach is to connect a muffler or silencer to the exhaust port.
Generally speaking, pneumatic device-related mufflers attenuate noise by presenting a barrier to airflow, absorbing sound waves, or both. For most commercial applications, a typical pneumatic muffler includes a cylindrical housing configured for mounting to the exhaust port. The housing defines one or more internal chambers through which air from the exhaust port is directed. Further, an airflow barrier and/or sound absorption insert is normally disposed within the housing. Finally, the housing normally forms one or more airflow passages or apertures through which air is released (or exhausted) from the muffler. A wide variety of materials are available for use as the insert, ranging from metals and cloth to composite materials. For example, various pneumatic muffler products are available from Minnesota Mining & Manufacturing Company of St. Paul, Minn. that make use of a replaceable acoustic barrier insert.
Regardless of the exact configuration, two important parameters must be considered when assessing pneumatic muffler performance. First, the muffler must limit exhaust noise to an acceptable level. Additionally, any back pressure caused by the muffler must be accounted for. In simplest terms, a portion of the total system pressure is required to push a given airflow through the muffler. This pressure is referred to as the "back pressure" of the muffler. Depending upon the particular application and level of back pressure, overall performance of the pneumatic device may be greatly diminished.
It is well known that noise attenuation and back pressure minimization are inversely related. That is to say, the noise reduction characteristic of a particular pneumatic muffler may be enhanced by incorporating additional, or a more dense, insert material. However, this additional material or material density will likely increase back pressure, thereby diminishing muffler usefulness. With this relationship in mind, noise attenuation and back pressure can be optimized by designing the muffler housing and associated insert material to be relatively large. For example, most commercially available pneumatic mufflers have a length in the range of 4-8 inches (102-203 mm) and an outer diameter in the range of 1.5-4 inches (38-102 mm). To maximize airflow from the muffler (and therefore minimize back pressure), the pneumatic muffler housing typically includes a series of circumferential slots along the housing side wall. Thus, the housing itself normally serves as only a partial barrier to airflow and sound waves.
Pneumatic mufflers adhering to the above-described dimensional characteristics have proven to be highly effective in attenuating pneumatic exhaust noise with minimal back pressure. Unfortunately, however, certain pneumatic device applications do not provide sufficient clearance for mounting of these relatively large mufflers. For example, certain types of processing equipment (e.g., a mail sorter) include a valve bank incorporating a large number of pneumatic valves (and thus exhaust ports) positioned in close proximity to one another. Often times, the valve exhaust ports have a center-to-center spacing of less than 1.5 inches (38 mm). Obviously, the above-described "standard" muffler sizes prohibit their use with these limited clearance applications, as it is impossible to mount two of the mufflers side-by-side. Further, where the muffler housing is relatively long and extends an appreciable distance from the pneumatic device, the opportunity for an operator to inadvertently contact and possibly break or otherwise damage the muffler becomes increasingly prevalent.
Efforts have been made to overcome the clearance problems associated with closely spaced pneumatic valve exhaust ports. For example, tubing can be connected to each of the exhaust ports and then routed to a single muffler at a location spaced from the exhaust ports. This technique is expensive and time consuming, and likely results in prohibitive back pressure. Alternatively, attempts have been made to produce a reduced-sized cylindrical muffler housing containing a barrier material such as sintered brass or felt. While a series of these so-configured mufflers can be mounted side-by-side to a confined clearance valve bank, the necessarily small volume of selected insert material associated with each of the individual mufflers cannot alter airflow and/or absorb noise to provide sufficient noise reduction. Of particular concern are relatively continuous valve cycling applications. Often times, these devices require a relatively small noise reduction (e.g., in the range of 5 dB for an open exhaust noise level of 90 dB) per exhaust port, but are highly sensitive to back pressure. The commercially available, reduced-sized mufflers may provide for potentially acceptable noise reduction, but may generate an extremely high back pressure, and therefore cannot be used.
Mufflers for use in attenuating noise produced by pneumatic devices continue to be extremely popular. However, where the particular pneumatic device has very limited clearance space for receiving the muffler, "standard" sized mufflers cannot be used. Efforts to design a viable, reduced-sized pneumatic muffler have been unavailing. Therefore, a need exists for a pneumatic muffler having acceptable noise reduction and back pressure characteristics that is sized for use with restricted clearance space applications.